1. Field of the Invention
The present invention relates to measuring moisture content of non-condensible gases and vapors amperometrically, and, more particularly, to a solid polymeric eletrolytic hygrometer.
2. Description of the Prior Art
Hygrometers, which measure the humidity of a gas, may be characterized as being of two types: relative and absolute. The relative type, such as wet and dry bulb thermometers, require calibrations under various test conditions, are limited in dynamic range, especially in low moisture concentration environments. The absolute type, such as the electroyltic cell, have a great dynamic range in measurable moisture concentrations, as well as a great temperature and pressure range. The electrolytic cell gives an exact measure of the available water by elecrolyzing it to hyrogen and oxygen.
Current state-of-the-art hygrometers use the principles disclosed by F. A. Keidel in Anal. Chem. 31, No. 12, p. 2043, Dec. 1959. This hydrometer design consists of an electrolytic cell through which moist gases are drawn. The water vapor content of the gas is absorbed at the surface of a polyphosphoric acid film and electrolyzed to gaseous hydrogen and oxygen by means of closely-spaced noble metal electrodes.
The acid electrolyte in these hydrometers is a complex inorganic polymer of phosphorous pentoxide and meta-phosphoric acid that is a viscous liquid at ambient temperatures and has a vapor pressure estimated near 0.1 mm Hg. In accordance with Faraday's Law, the electrolysis of one gram equivalent weight of water (9.01g) theoretically requires 96,500 colombs of electricity. The voltage of the cell must be greater than 1.23V and in most cases is about 10 times this value. The current drawn by the electrolytic cell is a direct measure of the rate at which water is being electrolyzed, which in equilibrium conditions equals the rate at which water is being absorbed at the acid electrode interface. A knowledge of the gas flow rate and the current drawn by the cell gives an absolute and continuous measure of the humidity mixing ratio of the sampled gas.
A known problem area in the use of polyphosphoric acid as an electrolyte in a hygrometer sensor is that the material is a liquid that can move under high g loads, and would tend to evaporate, particularly under prolonged exposure to the hard vacuum of deep space. In addition, certain gases, such as ammonia, interfere with the absorption process and thus degrade the sensor operation.
Other hygrometers have been disclosed which utilize solids such as polystyrene to absorb water from an air sample. All disclosed solids can be oxidized, so that they decompose rather quickly and must be replaced frequently.